Current best commercial practice in manufacturing steel strip and sheet is the continuous casting of thin slabs. These slabs are then hot-rolled to a thickness which can be coiled, so-called "Hot Band". Hot Band is subsequently de-scaled of its oxidized surfaces by pickling in an acid bath or by shot blasting. Following this operation, the de-scaled band is cold-rolled to usable commercial gauges. Non-ferrous metals are processed in similar fashion from slab or billet without the need for de-scaling.
Efforts to avoid the hot-mill portion of the above procedure have led to thin strip casting processes. These processes, in general, have cast surfaces which lack the fine quality required by the manufactured products in which they are used even after further rolling. Steel sheet, in particular, not only needs a fine surface but in many final uses must also be protected -from destructive corrosion by coating or plating the surfaces with protective metal coatings.
The current commercial practice of hot-rolling and then cold-rolling to final usable gauge, in addition to hot-mill costs, gives rise to an additional problem. This problem is known as "shape". The many rolling passes required to reduce the slab to final usable gauge results in distortions to the surface caused by variations in thickness across and along the length of the strip. These variations are caused by roll deflection and temperature variations during processing.
The present invention resolves the problem of surface quality by casting molten metal between two moving bands of cold-rolled strip which already possess the requisite surface quality. Fears by the steel industry that their cast strip would have large grain which could not be reduced sufficiently by the few roll passes required to reach usable gauges have proven to be without foundation. The cast grain size in thin cast strip has proven to be much smaller than anticipated and capable of being reduced to the fine grain necessary for strength and formability. Shape problems are minimized by reducing the number of roll passes required to obtain usable cold-rolled gauges.
Inherent in the casting process of this invention is the ability to coat or plate the cast strip directly with other metals. Thus, the final rolled product is ready for use in corrosive applications without further coating or plating. Current methods for coating or plating involve hot dipping the rolled product in molten metal or electro-plating it with other metals.
The coating made possible by this invention is of uniform thickness and variable to meet the required specification for type and thickness of the final product coating. The current hot dip process results in a variable coating thickness. Electro-plating is generally not economic for thicker coatings.
Approximately 10% of finished cold-rolled product is used in producing the cast product in this invention. This is a very small process cost to pay to eliminate the hot-mill and de-scaling necessary in current production of cold-rolled strip and sheet. The ability to coat while casting and the improved quality of that coating provides a welcome bonus for the industry.